From ESA, “Welcome to a Comet”: “Philae’s view of the cliffs at Abydos. One of the lander’s three feet can be seen in the foreground.” Image Credit: ESA/Rosetta/Philae/CIVA

It would have been a heroic coda to an already ambitious, historic mission, but alas, it was not meant to be. For the past few months, controllers at Germany’s DLR (the German Aerospace Center) have been sending commands to Philae, Rosetta’s lander, which touched down not once, but three times upon the surface of Comet 67P/Churyumov-Gerasimenko in November 2014. In June 2015, they miraculously did manage to get short bursts of data from the plucky refrigerator-sized lander. However, at present time, attempts to contact and once again “reawaken” Philae have been labeled in vain as the comet moves farther away from the Sun, creating conditions not optimal for lander survival. Stephan Ulamec, DLR’s Philae project manager, stated this week that the team is no longer sending commands: “The chances for Philae to contact our team at our lander control center are unfortunately getting close to zero.”

Cometary Conditions Make Contact Next To Impossible

In August last year, Comet 67P approached perihelion, its closest point to the Sun. During that time, the comet grew more active, blasting out emissions of dust, water, and gas. In a bid to protect itself, the Rosetta orbiter had to be moved farther away from the comet’s nucleus. These maneuvers unfortunately prevented optimal communication with Philae, which had provided seven data transmissions between June 13 and July 9 (it is believed the lander awakened in April, but was unable to make communications until June, when it had warmed up sufficiently).

The European Space Agency (ESA) reported that despite sending commands, no further communications have been received, and attempts to send blind commands have also not managed to raise Philae. Other factors potentially hampering communication include failures of the lander’s transmitters and receivers, dust accumulating on its solar panels, and even the possibility Philae may have “shifted” on the comet’s surface due to its activity.

The fact that Philae even landed and completed most of its science package is nothing short of a miracle. On Nov. 12, 2014, the lander, meant to touch down and make its cometary home at a site called Agilkia, made three separate touchdowns as it failed to deploy its landing harpoons, meant to secure itself to Comet 67P’s surface (it also bears mention that Philae had a faulty thruster). It eventually touched down in a site called Abydos, which was surrounded with craggy cliff-like formations. Despite its arduous journey, Philae completed 80 percent of its science package in the 64 hours it survived initially on the comet’s surface, even working as it “sailed” across its surface.

ESA underscored Philae’s unprecedented achievements during its short lifetime: “In the 64 hours following its separation from Rosetta, Philae took detailed images of the comet from above and on the surface, sniffed out organic compounds, and profiled the local environment and surface properties of the comet, providing revolutionary insights into this fascinating world.” The lander then went into hibernation due to lack of sunlight, reawakening briefly months later.

Video Credit: European Space Agency on YouTube

Despite its perhaps terminal sleep, scientists on Earth are still keen on tracking down Philae. Sylvain Lodiot, ESA’s Rosetta spacecraft operations manager, said: “The comet’s level of activity is now decreasing, allowing Rosetta to safely and gradually reduce its distance to the comet again. Eventually we will be able to fly in ‘bound orbits’ again, approaching to within 10-20 km — and even closer in the final stages of the mission — putting us in a position to fly above Abydos close enough to obtain dedicated high-resolution images to finally locate Philae and understand its attitude and orientation.” Matt Taylor, ESA’s Rosetta’s project scientist, added, “Determining Philae’s location would also allow us to better understand the context of the incredible in situ measurements already collected, enabling us to extract even more valuable science from the data.” While controllers are no longer sending commands to Philae, they will still “listen out” for possible transmissions.

Rosetta Orbiter To Continue Science Mission Through September

Rosetta’s orbiter is still going strong; recently, it was announced that its VIRTIS infrared instrument uncovered evidence of water ice on the comet’s surface. In June last year, ESA announced that the Rosetta mission would be extended through September 2016. This extension will allow the orbiter to make scientific observations of the comet as it moves farther and farther away from the Sun in its orbit. At present time, Comet 67P is headed back out to the outer solar system, some 350 million kilometers (217 million miles) from the Sun. As Rosetta gets farther away from sunlight, its solar panels will be unable to power the spacecraft sufficiently.

However, like its lander, Rosetta won’t just fade away into obscurity. According to the journal Nature, controllers intend to impact the orbiter into Comet 67P. While Rosetta will capture fantastic high-resolution images prior to impacting, it will also make observations in its final moments.

The journal stated in a November article: “Although Philae sent back some data during its descent, Rosetta has more powerful — and more varied — sensors and instruments. The orbiter will also descend much more slowly than Philae did, allowing it to gather more data and better pictures.”

“Once it gets to 4 kilometers, for example, Rosetta should be able to distinguish between the gases emerging from each of the duck-shaped comet’s two lobes to determine whether the regions vary in composition,” says [Kathrin] Altwegg, who leads the team behind ROSINA (the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis). “That could shed light on the environments in which each was formed.” Altwegg also pointed out, more poignantly, that Rosetta’s end will be a particularly fitting one: “This way Rosetta gets to live happily ever after on the comet with Philae.”

Rosetta’s mission originated in the early 1990s, and was launched from Kourou, French Guiana, aboard an Ariane 5 launch vehicle in 2004. While its end will inspire emotion, the mission will leave a legacy of cometary science for scientists and researchers to sift through for years, perhaps decades, to come.

Picture this scenario:
-Rosetta lands on Chury, makes some photos and does other scientific research
– as the comet gets too far away from the sun, the solar arrays deliver insufficient power and Rosetta turns off
– Chury runs 3/4 of a round around the Sun and is back where the Sun’s rays feed the solar array again
– the array charges the batteries of Rosetta
The question is: would Rosetta then wake up and greet us again?